A variety of transcription factors are involved in the regulation of expression of proteins during embryogenesis and adult stem cell maturation. Homeobox (HOX) genes contain a highly conserved nucleotide sequence of about 180 bp which encodes a homeodomain of about 60 amino acids. A homeodomain is a DNA-binding protein domain which can bind to target sequences in other genes and regulate their expression during development. The clustered Hox genes are key developmental regulators and are highly conserved throughout evolution. The homeotic HOX proteins which they encode share the 60 amino acid homeodomain and function as transcription factors to control axial patterning by regulating the transcription of subordinate downstream genes, e.g. developmental genes. In Drosophila and other insects there are eight different Hox genes that are encoded in two gene—complexes, while in vertebrates there are 39 genes organized in four complexes. The four gene complexes are assigned the letters A to D. Based on sequence similarities the genes can be sorted into 13 “paralog” groups. The order of the paralogs along the chromosomes are conserved in the four complexes. The gene name is obtained by concatenating the gene complex letter designation with the group number, e.g. HOXA1, HOXB4 etc.
Pre-B-cell transformation related gene (PBX) is also an important regulatory protein that controls gene expression during development by interacting cooperatively with HOX to bind to the target DNA (Mann et al., 1996, Trends Genet., 12(7), p 258-262). “Engrailed” proteins are also able to bind to PBX. PBX and HOX are known to interact via the hepta- or hexa-peptide region on the HOX molecule, which is highly conserved (Phelan et al., 1995, Mol. Cell. Biol., 15(8), p 3989-3997 and Neuteboom et al., 1995, PNAS, 92, p 9166-9170). The hexapeptide is separated by a linker region from the N-terminus of the homeodomain. Once PBX and HOX have bound to one another, they enter the nucleus of a cell and there bind to target DNA and repress or activate that target gene's transcription.
Whilst these proteins are known to be involved in embryogenesis their precise roles have not been elucidated. Over or under expression of HOX proteins gives rise to a variety of consequences in vitro which implicate the involvement of these proteins in the control of differentiation processes. However the consequences of perturbing the interaction between PBX and the co-factors to which it binds have not been examined. Furthermore, PBX:HOX binding antagonists have been found to be rather specific to specific forms of the protein binding partners involved (see Peltenburg & Murre, 1996, EMBO Journal, 15(13), p. 3385-3393). Furthermore, it is believed that the linker region between the homeobox and the hexapeptide is required for cooperative binding between PBX and HOX (Peltenburg & Murre, supra and Neuteboom et al., 1995, PNAS, 92, p 9166-9170).